Upon entering the tetragonal phase at \(x>x_{\rm c}\) , \(C_{\rm e}/T\)  exhibits a dramatic change not only in the superconducting state but also in the normal state. As represented by \(x\)  = 0.20 in Fig.REF (h), \(C_{\rm e}/T\)  increases with increasing slope in the normal state when approaching \(T_{\rm c}\)  from high temperatures. In addition, \(C_{\rm e}/T\)  exhibits a kink at \(T_{\rm c}\) , without showing a discontinuous jump. Such a continuous \(T\) -dependence at \(T_{\rm c}\)  is reminiscent of the BEC transition in the free Bose gas systems [1]} although it is too simple to employ the free Bose gas model to our system. We note that similar enhancement of \(C_{\rm e}/T\)  in the normal state is obtained in the recent calculations for the strongly interacting Fermi systems near the unitary limit in the BCS-BEC crossover regime [2]}. As the temperature is lowered below \(T_{\rm c}\) , \(C_{\rm e}/T(T)\)  for \(x = 0.20\)  decreases with increasing slope, in stark contrast to \(C_{\rm e}/T\) (\(T\) ) in the orthorhombic crystals as shown in Figs.REF (b)-(e). Below \(T_{\rm c}\) , \(C_{\rm e}/T\)  decreases nearly in proportion to \(T^{1/2}\)  with large residual \(C_{\rm e}/T\)  in the limit \(T \rightarrow 0\) , which is indicative for a large DOS at zero energy. The large residual DOS is consistent with the recent thermal conductivity and STS measurements [3]}, [4]}. Similar \(C_{\rm e}/T(T)\)  is also observed for \(x\)  = 0.18, and 0.21 in the tetragonal phase as shown in Figs.REF (g), and (i). These results together with the sharp change in \(T_c\)  across \(x_{\rm c}\)  demonstrate that the superconducting properties in the tetragonal phase are drastically different from those in the orthorhombic phase.
